24.1 introduction to carbohydrates - · pdf file•carbohydrates are not true hydrates....
TRANSCRIPT
• Carbohydrates (sugars) are abundant in nature:
– They are high energy biomolecules.
– They provide structural rigidity for organisms (plants, crustaceans, etc.).
– The polymer backbone on which DNA and RNA are assembled contains sugars.
• The term, carbohydrate, evolved to describe the formula for such molecules: Cx(H2O)x.
• Carbohydrates are NOT true hydrates. WHY?
24.1 Introduction to Carbohydrates
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-1
• Carbohydrates (sugars) are polyhydroxy aldehydes or ketones.
– Consider glucose, which is made by plants:
– Describe the potential energy change that occurs during glucose photosynthesis.
– Is glucose a polyhydroxy aldehyde or ketone?
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-2
• Saccharides have multiple chiral centers, and they are often drawn as Fischer projections.
– Designate each chirality center in glucose as either R or S.
24.2 Classification of Monosaccharides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-3
• Saccharides have multiple chiral centers, and they are often drawn as Fischer projections.
• What does the suffix, “ose” mean?
• Define the following terms:
– Aldose and ketose
– Pentose and hexose
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-4
• Glyceraldehyde is a monosaccharide with one chirality center.
– Natural glyceraldehyde is dextrorotatory (D): it rotates plane polarized light in the clockwise direction.
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-5
• Naturally occurring larger sugars can be broken down into glyceraldehyde by degradation.
• Such sugars are often called D-sugars.
24.2 Classification of Monosaccharides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-6
• Recall that dextrorotatory versus levorotatory rotation cannot be predicted by the R or S configuration.
• Here, D no longer refers to dextrorotatory. Rather it refers to the R configuration at the chiral carbon farthest from the carbonyl.
24.2 Classification of Monosaccharides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-7
• There are four aldotetroses. Two are shown below.
• What are the other two structures?
24.3 Configuration of Aldoses
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-8
• Aldopentoses have three chirality centers. The number of isomers will be 23.
• Recall the 2n rule from Section 5.5.
• The D-sugars are naturally occurring.
24.3 Configuration of Aldoses
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-9
• Ribose is a key building block of RNA.
– WHAT is RNA? More detail to come in Section 24.10.
• Arabinose is found in plants.
• Xylose is found in wood.
24.3 Configuration of Aldoses
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-10
• Based on the 2n rule, how many aldohexoses are there?
• How many of the aldohexoses are D isomers.
• Glucose is the most common aldohexose.
• Mannose and galactose are also common.
24.3 Configuration of Aldoses
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-11
• Relevant ketoses have between three and six carbons.
• For each naturally occurring D isomer, there is an L enantiomer.
24.4 Configuration of Ketoses
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-12
24.4 Configuration of Ketoses
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-13
• Recall from Section 20.5 that carbonyls can be attacked by alcohols to form hemiacetals.
– The intramolecular reaction is generally favored for 5 and 6-membered rings. WHY?
24.5 Cyclic Structures of Monosaccharides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-14
• For the following compound, draw the mechanism and resulting product that results from acid catalyzed ring-closing hemiacetal formation.
24.5 Cyclic Structures of Monosaccharides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-15
• Monosaccharides, like glucose, can also undergo ring-closing hemiacetal formation.
• The equilibrium greatly favors the closed form called pyranose.
24.5 Cyclic Structures of Monosaccharides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-16
• Distinguish between the α and β anomers.
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-18
• Ketoses form both furanose (5-membered) and pyranose (6-membered) rings:
24.5 Cyclic Structures of Monosaccharides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-24
24.5 Cyclic Structures of Monosaccharides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-25
70% β 0.7% 23%-β
2% α 5%-α
• The equilibrium concentrations in water are above.
• The furanose form takes part in most biochemical reactions.
24.5 Cyclic Structures of Monosaccharides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-26
• Monosaccharides are generally soluble in water. WHY?
• To improve their solubility in organic solvents, the hydroxyl groups can be acetylated.
• WHY is pyridine added to the reaction?
• How might acetylation help in purification efforts.
24.6 Reactions of Monosaccharides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-27
• Monosaccharides can also be converted to ethers via the Williamson ether synthesis.
• Ether linkages are more robust than ester linkages. WHY?
24.6 Reactions of Monosaccharides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-28
• When treated with an excess of an alcohol, the hemiacetal equilibrium can be shifted to give an acetal.
• When a sugar is used, alpha and beta glycosides are formed.
24.6 Reactions of Monosaccharides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-29
Copyright 2012 John Wiley & Sons, Inc.
• The mechanism of glycoside formation is analogous to the acetal formation mechanism.
• Only the anomeric hydroxyl group is replaced.
24.6 Reactions of Monosaccharides
Klein, Organic Chemistry 1e 24-30
• The mechanism of glycoside formation is analogous to the acetal formation mechanism.
• What factors would you consider when trying to predict whether the alpha or beta anomer will be the major product?
• Practice with CONCEPTUAL CHECKPOINTs 24.28 and 24.29.
24.6 Reactions of Monosaccharides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-31
• Under strongly basic conditions, glucose and mannose interconvert.
• Mannose and glucose are epimers because they only differ in the configuration of one carbon center.
• Practice with CONCEPTUAL CHECKPOINT 24.30.
24.6 Reactions of Monosaccharides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-32
• Monosaccharides can be reduced to ALDITOLs shifting the equilibrium to the right. HOW?
– D-sorbitol or D-glucitol are sugar substitutes.
24.6 Reactions of Monosaccharides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-33
• If the sugar has an –OH attached to the anomeric carbon, then the sugar is a reducing sugar
• If it has –OR, then it is not a reducing sugar
Reducing sugars
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24 -34
• Practice with SKILLBUILDER 24.4.
24.6 Reactions of Monosaccharides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-37
• Disaccharides form when two sugars connect through a glycosidic linkage.
– The 1 4 glycosidic linkage is most common.
– The bottom ring is capable of mutarotation at its anomeric position.
– Because the anomeric position of the bottom ring is a HEMIACETAL rather than an acetal, it is in equilibrium with the open form. Thus, maltose is a reducing sugar.
24.7 Disaccharides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-42
• Cellobiose is similar to maltose. WHAT are the differences?
• Will cellobiose be a reducing sugar?
24.7 Disaccharides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-43
• Lactose is another disaccharide.
• Some people have trouble digesting lactose.
24.7 Disaccharides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-44
• Sucrose (table sugar) is also a disaccharide.
– Honey bees can convert sucrose into a mixture of sucrose, fructose, and glucose.
– Fructose is very sweet.
• Sucrose is not a reducing sugar. WHY?
24.7 Disaccharides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-45
• Cellulose is a polysaccharide containing 7000–12000 glucose units connected through glycosidic bonds.
• How is cellulose capable of giving plants like trees their rigidity and strength?
24.8 Polysaccharides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-46
• Starch is a major components of grains and other foods, like potatoes.
• What is the difference between molecules of starch and molecules of cellulose?
• Starch is made of amylose and amylopectin.
24.8 Polysaccharides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-47
• Amylopectin has some 16-α-glycoside branches.
• We can eat corn and potatoes, but not grass or trees. WHY?
24.8 Polysaccharides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-48
• Amino sugars like glucosamine are important biomolecules.
• Acetylated glucosamine can form an
important polysaccharide called chitin.
24.9 Amino Sugars
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-49
• The carbonyl groups in chitin allow for even stronger H-bonding between neighboring chains.
• Chitin is used in insect and arthropod exoskeletons. WHY?
24.9 Amino Sugars
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-50
• N-glycosides can be formed when sugars are treated with an amine and an acid catalyst.
• RNA and DNA incorporate important N-glycosides called nucleosides.
24.10 N-Glycosides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-51
• Ribose forms ribonucleosides in RNA.
• Deoxyribose forms deoxyribonucleosides in DNA.
24.10 N-Glycosides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-52
• There are four different heterocyclic amines that attach to deoxyribose molecules to form DNA nucleosides.
24.10 N-Glycosides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-53
• In DNA, the nucleosides are attached to phosphate groups forming nucleotides.
24.10 N-Glycosides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-54
• The phosphate groups of the nucleotides are connected together to make the DNA strand or POLYNUCLEOTIDE.
24.10 N-Glycosides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-55
• The nucleotides in DNA can attract one another through H-bonding of the DNA base pairs.
24.10 N-Glycosides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-56
• WHY does DNA form a double helix?
24.10 N-Glycosides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-57
• RNA is structurally different from DNA :
– The sugar in RNA is ribose. WHAT is the sugar in DNA?
– RNA contains uracil instead of thymine.
• RNA translates the information stored in DNA into working molecules (proteins and enzymes).
24.10 N-Glycosides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-58
• RNA strands generally do not form double helices like DNA.
• RNA strands can fold into many different shapes, and some even act as catalysts called ribozymes.
• It is possible that RNA evolved self-replication as an early step in the evolution of life from small molecules.
24.10 N-Glycosides
Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e 24-59